1. Field of the Invention
The present invention relates to methods and apparatuses for removing a protective coating from an optical fiber, and in particular to a method and apparatus for removing the protective coating from a silica or glass optical fiber without scoring or abrading the optical fiber, and further a method for inhibiting damage to the optical fiber.
2. Technical Background
Optical fiber manufacturers typically cover an optical fiber with one or more protective polymer coatings. Technicians routinely remove the outer protective coating from the optical fiber to make splices, attach connectors, pigtail the fiber to an optical component, or use the exposed portion of the optical fiber in fabricating an optical component. Removing or "stripping" the outer protective layer can be accomplished in a variety of ways, including contact stripping (such as mechanical or chemical processes) and non-contact stripping (such as a hot gas jet).
In many situations, mechanical stripping processes are generally preferred, particularly when the optical fiber must be stripped manually or in the field.
Concurrent with the advent of optical fiber, mechanical stripping tools similar in form and function to conventional wire-strippers were developed. In one example, a stripping tool having a deformable polymer or soft metal blade was suggested for use both with copper wire and acrylic optical fiber. Mechanical stripping tools generally provided a grooved or notched blade, with an adjustable diameter corresponding to that of the wire or optical fiber.
Glass or silica optical fibers quickly became the standard for fiber-optic communications, due to the exceedingly low attenuation or loss in the 1.3 nm and 1.55 nm wavelength transmission windows.
However, silica or glass optical fibers could be easily scored or abraded by mechanical stripping blades, resulting in weakening and breaking of fibers. Nicked or scored fibers could break due to tension or flexion well after connections were made, requiring significant time to track and repair the fault. Conventional stripping blades often damaged the optical fiber and reduced its tensile strength. For example, a coated optical fiber typically has a tensile strength of 600 to 800 Kpsi. Removing the polymer coating with a conventional fiber-stripping tool may cause the strength of the optical fiber to drop to 100 Kpsi or lower. Optical fibers having a tensile strength below 100 Kpsi are often considered unsuitable for use.
As such, significant emphasis has been devoted to increasing the precision and reliability of mechanical stripping equipment and processes.
One approach is to maintain strict tolerances for the alignment and position of the guides, gripping elements, stripping blades, and mandrels used in strippers. However, these tolerances can be difficult to monitor or adjust, and the blades must be maintained in optimal condition. Sharpening a blade perturbs the operational tolerances, as does using a dulled blade. In additional, accurate mechanical strippers do not conform easily to a variety of fiber diameters or coating types.
Consequently, other approaches have been employed in combination with mechanical stripping tools or equipment in order to lessen the required tolerances, as well as mitigate against damaging the glass optical fiber.
In particular, chemical-, thermal-, or radiation-softening processes have been developed to lower the hardness or modulus of the protective coating and permit it to be stripped more easily (thus allowing the blade of the stripping tool to remain displaced slightly from the surface of the glass optical fiber).
While functional, these processes suffer from several drawbacks, such as requiring additional equipment and supplies to operate, being more time consuming and labor intensive, allowing less portability and therefore less applicability for use in the field or outside a controlled manufacturing environment, and being more susceptible to variability in the softening process itself.
Similar techniques have also been employed to completely remove the protective coating from the optical fiber, but apart from non-contact stripping processes these chemical and thermal contact stripping processes can adversely affect the optical fiber, leave residues that degrade transmission, adhesion, or splicing, and have other undesirable side-effects. These adverse results similarly occur when the processes are used in combination with a mechanical stripping blade.
For example, chemical stripping involves using chemicals such as methylene chloride or hot concentrated sulfuric acid. This approach does not provide sufficiently precise control over the amount or depth of coating stripped from the optical fiber, or the affect of the chemical on the optical fiber itself. The chemical often removes the coating outside the desired stripping area because it cannot be prevented from flowing along the optical fiber underneath the coating. Once the coating has been stripped, chemical residue remaining on the optical fiber prevents the optical fiber from being coated again. Further, this chemical process cannot be used in the field because the chemicals are dangerous and hard to handle, technicians in the field lack adequate training or qualifications, and the chemicals require a significant amount of time (on the order of thirty minutes) to remove the coating from a typical optical fiber.
Another approach to improving mechanical stripping of glass optical fibers has been to use a blade fabricated from a "softer" non-metallic material such as graphite. However, it is difficult to characterize the relative "softness" of graphite and reproduce blades of uniform quality, since graphite is a general term covering a large range of carbon structures having various physical properties, including hardness. If a graphite composition of requisite softness is selected, it may be difficult to maintain an accurate edge on the blade without frequent sharpening and adjustment. The edge of the blade itself may be subject to damage which might not be visible to the technician, but which would increase the potential risk of damaging the fiber. In addition, while graphite may be considered "soft" at a macroscopic level, it is relatively "hard" at the microscopic level, having a crystalline-like structure that cleaves to form very sharp, acute hard edges which can micro-score or abrade the optical fiber, again resulting in the same weakening and breakage potential as a conventional hard metal blade.
Conventional mechanical fiber-stripping tools thereby create an unacceptable reliability problem, since they periodically produce optical fibers having a reduced tensile strength that is unsuitable for use. Though testing the tensile strength of the stripped optical fibers could diminish the reliability problem, technicians often cannot perform such testing. And even if unsuitable optical fibers could be reliably identified, the conventional fiber-stripping tool remains disadvantageous because it wastes the time needed to test and restrip the fiber (as well as the fiber itself).
Furthermore, in addition to the disadvantages discussed above regarding chemical and thermal stripping techniques, the conventional fiber-stripping tools and processes do not accommodate for preventing damage to the optical fiber after the protective coating layer has been removed. The bare optical fiber can be damaged by incidental contact, or even airborne particulates that lodge on its surface and create flaws. The optical fiber can also be damaged if moisture is allowed to contact the surface of the optical fiber.